I find in German, Hornblende is the mineral (equivalent to Amphibole), and Hornblendefels is rock consisting of that mineral only.

I also find that slate shale are both translate to Schiefer. The German Wiki even mentions the language difference:

Im Englischen wird zwischen dem unstrukturierten Sedimentgestein (shale) und dem metamorphen Produkt (slate) unterschieden; letzteres ist der Schiefer im engeren Sinne.

About methamorposed and gneiss, the 'methamorphosed' applies both to Gneis and Schiefer in that sentence, so it's not redundant.

_{*Lunatic*, n.
One whose delusions are out of fashion.}